It is known to employ infrared (IR) emitters in connection with dispersive and non-dispersive-type infrared spectrometers and gas analyzers. Applications are manifold, including medical and industrial gas detection and analysis. IR sources are employed in the carbon dioxide breath analysis of medical patients, alcohol breath analysis by law enforcement personnel, refrigerant gas leak detection, CO analysis for car exhaust, and so forth. In the various settings, the infrared source is typically a laser, a Nernst or Globar radiator, a semiconductor IR emitting diode, or electrically heated elements, such as wires or foil strips, which emit radiation in the infrared portion of the electromagnetic radiation spectrum. In most analytical systems, means are generally employed to modulate the infrared emission, including mechanical means such as a chopper wheel, whereby the emission is allowed or obstructed according to a predetermined cycle. The modulated beam passes through a sample volume that may include wavelength selective filters and strikes a photo detector which amplifies and conditions the signal for display.
Exemplary and illustrative devices in the prior art include those disclosed and taught in several recently issued U.S. patents, including: U.S. Pat. No. 4,859,859, to Knodle, et al, which discloses an infrared source comprising an electrically modulated, stable thick film mounted on a thin ceramic substrate. The source has good modulation but is limited to very low power.
U.S. Pat. No. 5,342,951 to Kocache et al teaches a miniature source of modulated infra-red energy comprising a very thin heated foil strip of low mass, mounted in a tensioned state by means of two resilient members, and a reflector positioned behind the wire to enhance the radiated energy. The source is hermetically sealed in an inert atmosphere with a cap with a plastic sealed window. The cap has an aperture through which the energy emerges, and which is sealed either with a window or a filter if a specific wavelength emission is desired.
U.S. Pat. No. 5,438,233 to Boland, et al, teaches an infrared lamp having a one-piece hollow body, a tungsten filament that emits broad band IR radiation, an optical window having an aperture for the radiation emission, an indium allow junction between the window and the body to seal the window, a pliable coating overlaying the indium junction and part of the body, and a gas contained within the body to prevent deterioration of the filament. While this device advances significantly over the prior art, an indium seal is fragile and easily broken, and any elastomer (i.e., pliable) coating employed to reduce the fragility will interfere with the use of a heat sink to control the temperature of the lamp. Finally, this lamp, to the knowledge of the present inventor, cannot be pulsed at frequencies useful for nondispersive IR gas analysis.
U.S. Pat. No. 5,838,016 to Johnson shows an IR radiation source comprising a radiation filament having a textured surface produced by seeded ion bombardment of a metal foil which is cut to a serpentine shape and mounted in a windowed housing. Specific ion bombardment texturing techniques tune the surface to maximize emissions in the desired wavelength range and to limit emissions outside that narrow range, particularly at longer wavelengths.
U.S. Pat. No. 5,864,144 to Laine, discloses an infrared (IR) radiation emitting device comprises a sinuous foil resistive element which emits IR radiation when heated by an electric current. The device is configured so that the resistive element emits radiation with a spatial intensity which substantially possesses a single axis. The resistive element may, for example, be configured as a substantially planar spiral.
U.S. Pat. No. 5,939,726 to the present inventor, discloses a pulsable infrared radiation (IR) source which can be used in nondispersive infrared gas analyzers. The pulsable IR source includes a narrow strip of thin metallic foil vertically mounted to the opposite sides of pins of a small transistor type (TO-5 or TO-8) header at the base of a large diameter deep reflector. Infrared radiation from both sides of the heated foil is directly reflected to form a highly efficient source. The foil can be oxidized or otherwise coated under controlled conditions to maximize its emissivity at operating temperatures up to approximately 1200 degrees K. The package is resistance welded with a cap and hermetically sealed window of a suitable material and filter as required to transmit the desired infrared spectrum. An appropriate inert backfill gas prevents further oxidation and enhances the pulsing capability.
U.S. Pat. No. 6,297,511 to Syllaios et al, teaches a high frequently infrared radiation emitter. The emitter includes a low-thermal-mass resistive membrane suspended by long thermal isolation arms over a substrate. The membrane is suspended over the substrate such that a resonant emitting cavity is formed between the membrane and the substrate. The low-mass, thermally isolated membrane design maximizes the temperature change induced by Joule heating of the resistive membrane and allows the emitted IR radiation to be modulated at high frequencies.
The foregoing patents and prior art devices disclosed and, to the knowledge and understanding of the present inventor, all other devices in the prior art, provide IR sources of either limited power or limited pulsability and modulation depth. For example, Cal-Sensors, Inc., of Santa Rosa, Calif., produces an IR emitter that is limited to no more than 727 degrees C. for its pulsable NiCr foil emitters and 900 degrees C. for its Kanthal wire coil steady state sources. Ion Optics, Inc., of Waltham, Mass. manufactures IR emitters limited to approximately 600 degrees C. The radiated power increases as the fourth power of the absolute temperature; thus, increasing the temperature of the filament is the best way to increase the output power. However, NiCr has an extremely short life span when operated above 800 degrees C. Thus the need for a durable and long lasting IR emitter having higher power and greater modulation depth has not been adequately addressed.
Reference to, and discussion of, the background art is intended to aid in discharging Applicant's acknowledged duty of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the above-indicated patents disclose, teach, suggest, show, or otherwise render obvious, either singly or when considered in combination, the invention described and claimed herein.